The field of the invention is of circumferentially wrapped prestressed structures, and their construction, which structures can be used to contain liquids, solids or gases. The invention is particularly useful in the construction of domed prestressed structures.
There has been a need for the improved construction of these types of structures, as conventional construction has proven difficult and costly. Many of these structures have had problems with stability and leakage, in part, due to the high pressures exerted by certain of the stored fluids and cracking due to differential dryness and temperature. Because of these deficiencies, many have required substantial wall thickness or other measures to contain the fluids, requiring inordinately high-costs for their construction. Furthermore, these structures generally do not lend themselves to automation.
Certain of these conventional structures have utilized inflated membranes. Indeed, inflated membranes have been used for airport structures where the structure consists of the membrane itself. Inflated membranes have also bee used to form concrete shells wherein a membrane is inflated and used as a support form. Shocrete, with or without reinforcing, is sometimes placed over the membrane and the membrane is removed after the concrete is hardened.
Another form of construction is exemplified by conventional "Binishell" structures. These are constructed by placing metal springs and regular reinforcing bars over an uninflated lower membrane. Concrete is then placed over the membrane and an upper membrane is placed over the concrete to prevent it from sliding to the bottom as the inflation progresses. The inner membrane is then inflated while the concrete is still soft. After the concrete has hardened, the membranes are typically removed.
A major drawback of the afore-described conventional structures is the high cost connected with reinforcing and waterproofing them for liquid storage. Moreover, with regard to the "Binishell" structures, because of the almost unavoidable sliding of the concrete, it is difficult if not impossible to avoid honeycombing of the concrete and subsequent leaks. As a result, these structures have not been very well received in the marketplace and have thus far not displaced the more popular and commercially successful steel, reinforced concrete and prestressed concrete tanks and containment vessels, which we now discuss.
In the case of prestressed concrete tanks, prestressing and shotcreting are typically applied by methods set out in detail in my U.S. Pat. Nos. 3,572,596; 4,302,978; 3,869,088; 3,504,474; 3,666,189; 3,892,367 and 3,666,190 which are incorporated herein by reference. As set forth in these references, a floor, wall and roof structure is typically constructed out of concrete and conventional construction techniques. The wall is then prestressed circumferentially with wire or strand which is subsequently coated with shotcrete. The machinery used for this purpose is preferably automated, such as that set forth in the above patents. Shotcrete is applied to encase the prestressing and to prevent potential corrosion.
The primary purpose for prestressing is that concrete is not very good in tension but is excellent in compression. Accordingly, prestressing places a certain amount of compression on the concrete so that the tensile forces caused by the fluid inside the tank are countered not by the concrete, but by the compressive forces exerted by the prestressing materials. Thus, if design considerations are met, the concrete is not subjected to the substantial tension forces which can cause cracks and subsequent leakage.
Major drawbacks of the above prestressed concrete tank structure are the need for expensive forming of the wall and roof and for substantial wall thickness to support the circumferential prestressing force which places the wall in compression. Furthermore, cracking and imperfections in the concrete structure can cause leakage. Also, concrete tanks are generally not suitable for storage of certain corrosive liquids and petroleum products.
A second major category of tanks are those constructed out of concrete, and utilizing regular reinforcing in contrast to prestressing. These tanks are believed to be inferior to the tanks utilizing circumferential prestressing because, while regular reinforcing makes the concrete walls stronger, it does not prevent the concrete from going into tension, making cracking at even greater possibility. Typically, reinforcing does not come into play until a load is imposed on the concrete structure. It is intended to pick up the tension forces because, as previously explained, the concrete cannot withstand very much tension before cracking. Yet reinforcing does not perform this task very well because, unlike circumferential prestressing which preloads the concrete, there are not prestressing forces exerting on the concrete to compensate for the tension asserted by the loading. Moreover, as compared to prestressed concrete tanks, reinforced concrete tanks require even more costly forming of wall and roof, and even greater wall thicknesses to minimize tensile stresses in the concrete.
Another general category of existing tanks are those made of fiber-reinforced plastic. These fiberglass tanks have generally been small in diameter, for example, in contrast to the prestressed or steel tanks that can contain as many as 30 million gallons of fluid. The cylindrical walls are sometimes filament-wound with glass rovings. To avoid strain corrosion, (a not very well understood condition wherein the resins and/or laminates fracture, disintegrate or otherwise weaken) the tension in fiber reinforced plastic laminates is limited to 0.001 (or 0.1%) strain by applicable building codes or standards and by recommended prudent construction techniques. For example, the American Water Works Association (AWWA) Standard for Thermosetting Fiberglass, Reinforced Plastic Tanks, Section 3.2.1.2 requires that "the allowable hoop strain of the tank wall shall not exceed 0.0010 in/in." A copy of this standard is provided in the concurrently filed Disclosure Statement. Adhering to this standard means, for example, that if the modulus of elasticity of the laminate is 1,000,000 psi, then the maximum design stress in tension should not exceed 1,000 psi (0.001.times.1,000,000). Consequently, large diameter "fiber-reinforced plastic" tanks require substantially thicker walls than steel tanks. Considering that the cost of fiber-reinforced plastic tanks has been close to those of stainless steel, and considering the above strain limitation, there are believed to have been no large diameter fiber-reinforced plastic tanks built world-wide since fiberglass became available and entered the market some 35 years ago.
Another reason why large fiber-reinforced plastic tanks have not been constructed in the past, is the difficulty of operating and constructing the tanks under field conditions. Water tanks, for example are often built in deserts, mountaintops and away from the pristine and controlled conditions of the laboratory. Resins are commonly delivered with promoters for a certain fixed temperature, normally room temperature. However, in the field, temperatures will vary substantially. Certainly, variations from 32.degree. F. to 120.degree. F. may be expected. These conditions mean that the percent of additives for promoting the resin and the percent of catalyst for the chemical reaction, which will vary widely under those temperature variations, need to be adjusted constantly for the existing air temperatures. Considering that these percentages are small compared to the volume of resin, accurate metering and mixing is required which presents a major hurdle to on-site construction of fiber-reinforced plastic.
Turning now to the seismic anchoring aspects of the present invention, in conventional concrete tank construction, methods used to compensate for earthquakes and other tremors have includes built-up wall thicknesses, and seismic cables anchoring the walls of the tank structure to the footing upon which the walls rest. These seismic cables typically allow limited horizontal movement between the walls and footing in the hope of dissipating stresses. Since tanks typically rest on a circular concrete ring or footing reinforced with standard steel reinforcement, the seismic cables are encased in the concrete footing. In most instances, the seismic cables are encased in sponge rubber sleeves where they exit from the footing (also called a foundation) into the walls at angles varying from 30.degree. to 45.degree. with the horizontal surface of the footing. The other end of the seismic cables are then encased in the concrete walls of the tank. The walls of the tank typically rest on a rubber pad placed between the wall and the footing. This placement allows the walls to move radially in or out in relation to the footing to minimize the vertical bending stresses and strains caused by circumferential prestressing, filling or emptying of the tank, or by horizontal forces caused by earthquakes or other earth tremors. In many instances the cables connect the wall and the footing prior to the addition of circumferential prestressing. This earlier means to compensate for seismic and other forces can be seen by its very description to be very complex and ineffective especially for a given cost.